There has been an enormous interest in recent years in exploiting the uniformly sized ar shaped channels and cavities of zeolites and related crystalline molecuar sieves for shape-selective adsorbents and catalysts. Unfortunately, the range of potential substrates and/or reactions are limited by the types of active site functionalities that may be currently engineered into these systems.
Most of the difficulty appears to be related to the small size of the micropores. While playing an essential role in the shape selectively of zeolites and relate molecular sieves, the size restriction also limits the ability to modify the micropores using post-synthetic procedures. For example, in a typical scheme, the free silanol groups dispersed throughout the structure are targeted for further functionalization. However, because exterior surface silanols are much more accessible than those in the interior, most of the functionalization occurs at these exterior sites where shape selectivity is generally not possible.
In another context, organic structure-directing agents ("SDAs") that contain silicon alkoxides that are covalently attached have been prepared. These organosilanes are used to direct the formation of molecular sieves. In this case, the organic component completely fills the void space. Removal of the organic is the only way to create void spaces in these materials and this yields only an inorganic solid. Hence, using organosilanes as structure-directing agents is not a viable vehicle for functionalization since the SDAs' removal would also eliminate the intended functionality.
As a result, new strategies are required for incorporating functional groups into zeolites and related molecular sieves to fully exploit their potential as shape-selective adsorbents and catalysts.